水力割缝空间分布模式对煤层卸压增透的作用规律

The effect of spatial distribution mode of hydraulic slotting on pressure relief and permeability enhancement of the coal seam

水力割缝技术是实现煤层卸压增透的有效手段,目前由水力割缝技术形成的致裂裂缝空间分布模式对煤层卸压增透的作用规律尚不明确。本文采用颗粒流模拟方法(PFC2D)对内含不同角度单缝及不同空间排布方式多缝的煤体开展了单轴压缩数值模拟试验,针对水力割缝周围微裂缝大量发育与连通促使煤层卸压增透的物理机制,提出了评价煤层割缝卸压增透效果的2个指标:加载过程中微裂缝显著产生时的单轴应力门限(σγ)与多条割缝的连通性。其中,σγ越低、多条割缝的连通性越强,割缝间的微裂缝越容易在低应力条件下形成并相互连通,割缝的卸压增透效果越好。模拟结果表明,单条割缝与煤体边界最大主应力方向夹角(α)呈90°时γ最小(3.2MPa);2条割缝(α为90°)排布方向与煤体边界最大主应力方向(割缝排布角β)呈45°时,水力割缝间具有最高的连通性与较低的应力门限(σγ为2.4 MPa);3条割缝(α为90°且β为45°)呈折线型交错排布模式时,割缝间的连通程度最高,且σγ较双割缝进一步降低了16.7%。通过上述模拟结果,确定了有利于煤层卸压增透的割缝最优空间分布模式,即α为90°的多条割缝以45°的排布角(β)交错分布。

The hydraulic slotting technology is an effective means to relieve the pressure and enhance the permeability of the coal seam. However, at present, understanding of the effect of spatial fracture distribution produced by hydraulic slotting on the pressure relief and permeability enhancement in coal seam remains unclear. In this paper, the particle flow code(PFC2 D) has been used to carry out numerical simulating test of the uniaxial compression on the coal body with single cracks at different angles and multiple cracks with different spatial distributions. In view of the physical mechanism of the nucleation and coalescence of microcracks surrounding the slotting crack enhancing the pressure relief and permeability of the coal seam, two indexes, uniaxial stress threshold when micro-cracks are produced significantly during the loading process and the connectivity of multiple slotting cracks, are proposed to evaluate the enhancement effect of hydraulic slotting on pressure relief and permeability in the coal seam. The lower σγ is, the stronger the connectivity of multiple slotting cracks becomes, the easier the nucleation and coalescence of microcracks surrounding the slotting crack in coal with lower stresses, and the better the effect of the pressure relief and permeability. The simulation results have shown that σγ reaches the minimum(3.2 MPa) as the dip angle(α: the angle between the slot and the direction of the maximum principal stress) of a single slot reaches 90°. When the arrangement-angle of two slots(α =90°) and the maximum principal stress direction of the coal body(β: the angle between the arrangement direction of the two slots) attains 45°, the connectivity of the slotting cracks reaches the best with a more moderate stress-threshold(σγ=2.4 MPa). The interlaced distribution of slotting cracks corresponds to the best crack-connectivity with a lower σγ than that of two slots(2 MPa, reduced by 16.7%). The above simulation results show that the interlaced arrangement of multiple hydraulic slots(=90° and β=45°) is the best spatial distribution mode for pressure-relief and permeability improvement in the coal seam.